Microwave tube with shielded laddertype delay structure



Dec. 8, 1964 L. D. COHEN ETAL 3,160,781

MICROWAVE TUBE WITH SHIELDED LADDER-TYPE DELAY STRUCTURE Filed April 24. 1961 Fj'q. 1

LEO/M420 0. COHEN BY JZMES ,4. NOLA/V0 fi. 1. an

A T TOENE Y INVENTORS.

United States Patent 3,169,781 MHCRQWAVE TUBE WITH SHEELDED LADDER- TWE EELAY STRUCTURE Leonard 1). Cohen, Brooklyn, and lenses A. Noland, Hicksville, N.Y., assignors to General Telephone and Electronics Laboratories, Inc, a corporation of Delaware Filed Apr. 24, 1961, Ser. No. 195,666 7 Uaims. ((Jl. Sid-35} This invention relates to microwave tubes and in particular to microwave tubes for use at wavelengths of less than one centimeter.

In an article entitled Traveling Wave Tube Experiments at Millimeter Wavelengths With a New, Easily Built, Space Harmonic Circuit by A. Karp, ublished "m the Proceedings of the I.R.E., vol. 43, pp. 41-46, January 1955, there is disclosed a traveling wave tube designed for operation at millimeter wavelengths. The tube employs a slow-wave structure consisting of a ridged rectangular waveguide having a series of transverse slots formed by an array of spaced rectangular wire tapes in the broad wall opposite the ridge. A beam of electrons is directed against the end tape in the slotted wall. When the electron beam strikes this tape it is divided into two parts, each travelling parallel to opposite surfaces of the tape. An electromagnetic field developed across each slot modulates the beam.

While this tube performs well at ordinary beam current densities, operation has not been entirely satisfactory at higher current densities. Specifically, it has been found that when a high density electron beam strikes the end tape, the tape tends to expand and droop thus exposing the adjacent tape to the beam. The second tape then expands and sags as does each of the other tapes in turn causing the spacing between the tapes and the waveguide ridge to change. The change in spacing causes significant variations in the tuning characteristics of the tube. In addition, the direct interception of the beam by the tapes causes them to move in a random manner before an equilibrium condition is reached resulting in the generation of spurious signals and noise in the tube output. In some cases, the high beam current densities cause one or more tapes to burn out, thus lowering the power output of the tube and producing fluctuations in the output characteristic due to reflections.

Accordingly, it is an object of our invention to provide 1 a microwave tube of the type described which is suitable for use at wavelengths less than one centimeter and at high beam current densities.

Another object is to provide a microwave tube in which the slow-wave structure is not adversely affected by a high density electron beam.

Still another object is to provide a microwave tube which provides relatively high power output, high sensitivity and low noise.

A further object is to provide a microwave tube of the type described in which the slow-wave structure includes a protective element as an integral part thereof.

A still further object is to provide a microwave .tube in which the protective element may be easily and accurately aligned within the tube.

Yet another object is to provide a microwave tube employing a protective element which does not adversely afiect the interaction between the electromagnetic wave and the electron beam.

In the present invention, a microwave tube is provided in which a beam of electrons isproduced by an electron gun mounted within an evacuated envelope. One end of a waveguide extendinginthe direction of the beam is positioned adjacent the electron gun. A ridge, projecting from the inner wall of the waveguide, is located ICC parallel to and spaced from the electron beam. A conductive strip, or ladder, is positioned within the waveguide in the path of the electron beam, the surface of the conductive strip being parallel to the top of the ridge. The section of the strip adjacent the electron gun consists of a smooth, continuous thermally and electrically conductive protective sheet. The remainder of the strip consists of a plurality of spaced transverse slots defined by an array of spaced rectangular wire tapes.

When the electron beam strikes the protective sheet, the beam is split into two parts. The upper part of the beam travels along the'waveguide between the conductive strip and the top of the waveguide while the lower part travels between the strip and the ridge. The center part of the beam is intercepted by the continuous protective sheet thereby dissipating the beam energy which would otherwise be absorbed by the first tape in the strip. As a result, a high density electron beam can be used without damaging the tapes or disturbing their alignment.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1 is a schematic cross-sectional diagram of a tube employing our invention;

FIG. 2 is a cutaway perspective view of the slow-wave structure used in the tube of FIG. 1;

FIG. 3 is a plan view of the upper wall of wave structure shown in FIG. 2; and

FIG. 4 is an end view of the upper wall of the slowwave structure.

Referring to FIGS. 1 and 2, there is shown a microwave tube comprising an electron gun 10 and a slowwave structure 11 located in an evacuated metallic envelope 12. A permanent magnet or an electromagnet (not shown) is used in a conventional manner to focus the beam.

The slow-wave structure 11 includes a rectangular molybdenum waveguide block 13 having a tapered inner ridge l4 projecting from the inside of the lower broad the slowwall 15. A tapered outer ridge 16 projects from the other a side of wall 15. The upper wall 17 of slow-wave structure 11 consists of a frame 18 on which is wound rectangular wire tape 19. Frame 18 is provided with a centrally located longitudinal slot 20 and two smaller longitudinal slots 21. The entire slow-wave structure is mounted on a metallic base plate 22 and supported between metallic flanges 23 and 24. The envelope 12 and slow-wave structure 11 are electrically conductive and are maintained at a positive potential with respect to the electron gun ltlby a battery 25.

The upper wall 17 of the slow-wave structure 11 is illustrated in detail in FIGS. 3 and 4. As shown in FIG. 3, the portions of tape 19 that cross slots 20 and 21 form a conductive ladder structure 36) divided into a protective section 31 and an interaction section 32.

In winding the frame, pure gold wires are first insertedin the slots 21 of frame 18. A continuous length; of rectangular molybdenum tape 19 is then closely wound over the section 31 with the turns touching but not overlapping. After section 31 is wound, the same tape In a typical tube, section '31 extends for v attenuation.

=3 tion 31, the gold braze flows into the interstices between the tapes 19 thereby joining adjacent turns and producing a continuous sheet of thermally and electrically conductive material.

In operation, an electron beam produced by electron gun passes through the aperture 40 in flange 23 and impinges upon the end of protective section 31. The beam is split into two parts, one part traveling between the ridge 14 and the ladder tapes 19 and the other part traveling in slot 20 between the frame 18 and tapes 19. The center part of the beam is intercepted by the protective section 31. Since section 31 presents a continuous thermally conductive surface, the energy in the center part of the beam is dissipated and does not damage the tapes 19 located in the interaction section 32. V

In the pass band of the circuit, an RF field is developed across each of the transverse slots formed by the tapes 19 in section 32' of the ladder structure fit As the electrons in the beam pass the tapes 19, they are subjected to a relatively weak RF field, and when they pass the space between the tapes, they are subjected to a stronger field. As a result, the strength of the interacting compo nent of the RF field is alternatingly large and small along ladder 30 thus producing modulation of the electron beam.

In the tube shown, a backward wave is produced which travels from right to left toward flange 23, makes two right angle bends and then travels between the tapered ridge 16 of guide 13 and the base plate 22 toward flange 24. The output wave is connected to a coupler 59 through a tapered ceramic window 51.

Since the protective section 31 and interaction section 32 are both wound on the same frame, they are automatically in alignment and it is not necessary to separately align them during manufacture of the tube. Also, by making the thickness of the tape in the protective section the same as the thickness of the tape in the interaction section the electron beam is maintained in very close proximity to the tapes without touching them. Thus, eflicient coupling is obtained between the beam and the RF field. p

The protective section is sufficiently long to permit it to remain cool after beam interception. Consequently, the spacing between the tapes 19and ridge 14 remains constant and is not a function of the input beam power.

Also, the protective section is effectively a continuation of the broad wall of the waveguide and thereforedoes not introduce significant microwave reflections or As many changes could be made in thejabo ve construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1 l. A microwave tube comprising an electron gun, said electron gun producing an electron beam traveling along 7 a predetermined path; a waveguide positioned adjacent said electron gun and extending in the direction of said electron beam, said waveguide having a ridge located parallel to said electron beam and spaced therefrom, said ridge projecting from an inner wall of said Waveguide; and a conductive ladder element located within said waveguide in the path ofsaidrelectron beam, the surface of said ladder element near the edge adjacent said electron gun comprising a solid conductive sheet, the

remainder of said ladder element being providedwith a plurality of transverse slots, said solid conductive sheet splitting said electron beam into two parts, the energy in the central part of said beam being dissipated by said conductive'sheet. a

2. A microwave tube comprising an electron gun, said electron gun producing an electron beam traveling along a predetermined path; a waveguide positioned adjacent said electron gunand extending in the direction of said electron beam, said waveguide having a ridge located parallel to said electron beam and spaced therefrom, said ridge projecting from an inner wall of said waveguide; and a conductive ladder element located within said waveguide in the path of said electron beam, said ladder element including a solid conductive protective section forming an integral part of the portion of said ladder element adjacent said electron gun, the remainder of said ladder element being provided with a plurality of transverse spaced conductive tapes, said protective section splitting said electron beam into two parts, the energy in the center part of said beam being dissipated by said protective section.

3. A microwave tube comprising an electron gun, said electron gun producing an electron beam traveling along a predetermined path; a waveguide positioned adjacent said electron gun and extending in the direction of said electron beam, said waveguide having a ridge located parallel to said electron beam and spaced therefrom, said ridge projecting from an inner wall of said waveguide; and a conductive ladder element having its surface parallel to said ridge located within said waveguide in the path of said electron beam, said ladder element including a solid conductive protective section adjacent said electron gun and an interaction section provided with a plurality of transverse spaced conductive tapes, the thickness of said protective and interaction sections being equal, said protective section splitting said electron beam into two parts, the energy in the central part of said beam being dissipated by said protective section.

4. A microwave tube comprising an electron gun, said electron gun producing an electron beam traveling along a predetermined path; a waveguide positioned adjacent said electron gun and extending in the direction of said electron beam, said waveguide having a ridge located parallel to said electron beam and spaced therefrom, said ridge projecting from an inner wall of said waveguide; and a conductive ladder element having its surface parallel to said ridge located within said waveguide in the path of said electron beam, said ladder element including a solid conductive protective section adjacent said electron gun and an interaction section provided with a plurality of transverse spaced conductive tapes, the length of said protective section being approximately one-tenth the length of said ladder element and the thickness. of said protective andinteraction sections being equal, said protective sectionsplitting said electron beam into two parts, the energy in the central part of said beam being I dissipated by said protective section.

' of said waveguide block and extending in the direction beam into two parts, the energy in the central part of said beam being dissipated by said protective section.

6. A microwave tube comprising an electron gun-,said electron gun producing an electron beam traveling along a predetermined path; a rectangular waveguide block positioned adjacent said'electron gun and'extending in the direction'of said electron beam, saidwaveguide block having a ridge projecting from the inside surface of a broad wall thereof; the top of said ridge being spaced from said electron beam; a conductive frame affixed to said waveguide block above said ridge and extending in the direction of said electron beam, said frame having a centrally located longitudinal slot in the face thereof; a ladder structure including a protective section adjacent said electron gun and an interaction section, said protective section consisting of a plurality of conductively touching metallic tapes having the interstices between said tapes filled with a conductive material and said interaction section consisting of a plurality of spaced metallic tapes, said protective section intercepting said electron beam and splitting said beam into two parts, the energy in the central part of said beam being dissipated by said protective section.

7. In a microwave tube, a conductive frame aflixed to the upper surface of a ridged waveguide block, said conductive frame having a centrally located longitudinal slot in one surface thereof and ya pair of longitudinal wire positioning slots near the edges of said surface; a plurality of turns of metallic tape wound on said frame, said tape traversing the slots in the surface of said frame, the tape at one end of said frame being closely Wound to form a continuous conductive protective sheet having the interstices between the turns of said tapes filled with a concluctive material, the turns on the remainder of said frame being spaced from each other.

References Cited in the file of this patent UNITED STATES PATENTS Millman July 6, 1954 

7. IN A MICROWAVE TUBE, A CONDUCTIVE FRAME AFFIXED TO THE UPPER SURFACE OF A RIDGED WAVEGUIDE BLOCK, SAID CONDUCTIVE FRAME HAVING A CENTRALLY LOCATED LONGITUDINAL SLOT IN ONE SURFACE THEREOF AND A PAIR OF LONGITUDINAL WIRE POSITIONING SLOTS NEAR THE EDGES OF SAID SURFACE; A PLURALITY OF TURNS OF METALLIC TAPE WOUND ON SAID FRAME, SAID TAPE TRAVERSING THE SLOTS IN THE SURFACE OF SAID FRAME, THE TAPE AT ONE END OF SAID FRAME BEING CLOSELY WOUND TO FORM A CONTINUOUS CONDUCTIVE PROTECTIVE SHEET HAVING THE INTERSTICES BETWEEN THE TURNS OF SAID TAPES FILLED WITH A CONDUCTIVE MATERIAL, THE TURNS ON THE REMAINDER OF SAID FRAME BEING SPACED FROM EACH OTHER. 